TV4 - Sex linked crosses

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• Middelbare school
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• Leerjaar 4

Sex-linked inheritance

TV4

Ch 19

Inheritance

Multiple alleles

In some cases there are more than 2 alleles for the same gene. A good example of this is the AB0 blood groups. There are three alleles for this gene:

IA - This allele codes for antigen A on the cell membrane of red blood cells. 

IB - This allele codes for antigen B on the cell membrane of red blood cells. 

i - This allele codes for no antigens on the cell membrane of red blood cells.  

Alleles IA and IB are co-dominant

Both IA and IB are dominant over allele i

Which genotype(s) will lead to blood group B?

A person has blood group A. Which genotype can this person NOT have?

1. IAIA
2. IAi
3. IAIB
4. ii

Complete on paper and hand in picture:

A person with blood group 0 has a child with a person with blood group AB. What is the chance this child will have blood group A?

Complete on paper and hand in picture:
In a family both grandmothers have blood group 0 and both grandfathers have blood group AB.

Calculate the chance of their grandchild getting blood group B.

Learning objectives

You can distinguish between autosomal genes and sex-linked genes

You can explain how the X and Y chromosome determine sex

You can apply the laws of inheritance to sex-linked genes and crosses

Look at this karyogram.

Which conclusion(s) can you draw?

22 pairs normal chromosomes

- autosomes

1 pair sex chromosomen

- XX --> girl/woman

- XY --> boy/man

The X-chromosome holds way more genes than the Y-chromosome. 

Some disease and syndromes linked to the X-chromosome: 

- colour blindness

- hemophilia

- Duchenne muscular dystrophy

- deficiencies

Important gene on the Y-chromosome

SRY-gene: gene that regulates many other genes

If the SRY-gene is present a mammal embryo will develop into a male

The gene coding for the proteins in the cones on your retina that help you distinguish between red and green is on the X-chromosome.

How many alleles of this gene will a woman have? Only write down a number.

The gene coding for the proteins in the cones on your retina that help you distinguish between red and green is on the X-chromosome.

How many alleles of this gene will a man have? Only write down a number.

There is a difference in the number of alleles between men and women for genes on the X-chromosome (sex-linked genes). Therefore we use a different notation. 

XX for women

XY for men

On the X-chromosome we note in superscript which allele is present on that chromosome. Still a capital letter for dominant and small letter for recessive. 

You still write down the Y-chromosome to show it is a male, but it will not have an allele in superscript.

Complete on paper and upload a picture:

A heterozygous woman has children with a colourblind man. This form of colourblindness is sex-linked recessive. Draw the punnett square of this cross.

Use your punnett square from the previous question:

What is the chance of a colourblind child?

1. 0%
2. 25%
3. 50%
4. 100%

Use the same punnett square again

They get a daughter, what is the chance she is colourblind?

1. 0%
2. 25%
3. 50%
4. 100%

Use the same punnett square again:

They get a son, what is the chance he is colourblind?

1. 0%
2. 25%
3. 50%
4. 100%

Important points sex-linked inheritance: 

• Women have two X-chromosomes and because of that two alleles 
• Men have one X-chromosome and because of that one allele 
• It is possible that the chances of a particular trait are different for men and women 
• READ carefully what they are asking 
• DONT FORGET to also write down the X and Y chromosomes and not only the alleles 
• If the trait is recessive males are more likely to have it than females 
• If the trait is dominant females are more likely to have it than males 

Identifying sex-linked inheritance

Men only have 1 allele. Men ALWAYS receive their X-chromosome from their mom

The allele that fathers have they will always pass on to their daughters. 

In pedigrees with a sex-linked trait, the following rules apply:

- Dominant father = dominant daughters

- Recessive mother =  recessive sons

https:

Upload a picture of your answer:
The grey symbols are people with a disease that is caused by a sex-linked gene. The red symbols are not sick. 7 and 8 are having a child, 11. 11 turns out to be a girl. What is the chance she has the disease?

In Drosophila (fruit flies) there is a recessive allele that causes veinless wings. This gene is located on the X-chromosome. A female with veinless wings is crossed with a male with normal wings. The F1 offspring that come from this cross and used to cross with each other to form the F2 generation. Which percentage of the F2 females will have veinless wings?